function at(n) {
    // ToInteger() abstract op
    n = Math.trunc(n) || 0;
    // Allow negative indexing from the end
    if (n < 0) n += this.length;
    // OOB access is guaranteed to return undefined
    if (n < 0 || n >= this.length) return undefined;
    // Otherwise, this is just normal property access
    return this[n];
}

const TypedArray = Reflect.getPrototypeOf(Int8Array);
for (const C of [Array, String, TypedArray]) {
    Object.defineProperty(C.prototype, "at",
        {
            value: at,
            writable: true,
            enumerable: false,
            configurable: true
        });
}


var arr = ["sunwukong", "zhubajie", "tangseng"]

console.log(arr.at(-1)) // tangseng
console.log(arr.at(-2)) // zhubajie
console.log(arr.at(10)) // undefined 超出最大下标，那么就返回undefined，不会报错